The present invention relates to digital communications, and more particularly, to the calculation of parity bytes for frames of digital data in a synchronous communication system. The invention is advantageous for reducing the number of parity calculations that must be performed when data frames are multiplexed or similarly modified.
Synchronous communication systems organize digital data into frames having a predetermined format for transmission across a data link. It is often advantageous to use parity calculations for monitoring bit errors occurring in transmissions across the data link. In standard synchronous communication systems (e.g. SONET Bellcore GR-253 or SDH G.707), a bit interleaved parity byte, designated B2, is calculated for each basic data frame. The B2 byte calculation is generally calculated over nearly the entire frame and any changes to a frame""s data bytes necessitates the recalculation of the B2 byte. When the basic data frames are multiplexed into higher data rate frames, the transport overhead bytes are changed in certain frame locations and the associated B2 bytes must be recalculated.
Accordingly, there exists a definite need for a technique for efficiently implementing the recalculation of parity bytes in digital communication systems. The present invention satisfies these needs and provides further related advantages.
The present invention provides apparatus, and related methods, for recalculating parity bytes after multiplexing, or similar modification, of frames of digital data in a synchronous communication system. The parity byte calculation technique of the invention greatly reduces the number of parity byte calculations that must be performed when data frames are multiplexed.
The invention is embodied in a method for recalculating a parity byte value related to a frame of synchronous digital data after change to at least one data byte value in the frame from an original data byte value to a new data byte value when the parity byte""s original value resulted from exclusive OR calculations that included the original data byte value. The parity byte value is recalculated by performing, on the original parity byte value, exclusive OR calculations with the original data byte value and with the new data byte value to generate an updated parity byte value. The method of the invention is particularly advantageous for recalculating B2 parity bytes after multiplexing of, for example, four OC-12 SONET data frames into an OC-48 SONET data frame.
Another embodiment of the invention resided in a method, and related apparatus, for multiplexing four OC-12 signals into a single OC-48 signal of framed data bytes for synchronous communication over an optical fiber link. First, the four OC-12 signals are byte interleave multiplexed into the OC-48 signal. The B2 and M1 data bytes of frames of the OC-48 signal are replaced based on B2 and M1 data bytes of the four OC-12 signals. A new B2 byte is determined by performing, on the B2 byte""s initial value, exclusive OR calculations with the initial and resulting B2 bytes of the previous frame and further with the initial and resulting M1 bytes of the previous frame.
Other features and advantages of the present invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.